To achieve miniaturization and higher packaging density of memory cells of a nonvolatile semiconductor storage device, such as a NAND flash memory, the gate length is being reduced. To reduce the difficulty of processing, a flat cell structure is being adopted as an alternative to the conventional rocket cell structure. The flat cell structure adopts a silicon nitride film (SiN) instead of the conventional oxide film as hard mask films, from the viewpoint of etching selectivity in gate processing.
In the flat cell structure in which the silicon nitride film (SiN film) is used as hard mask films, however, the hard mask film on the select gate electrode is thicker than the hard mask film on the word line (control gate electrode) because of the difference in etching rate between the pattern of thicker lines and the pattern of thinner lines.
The hard mask film on the select gate electrode serves also as a stopper film in contact formation. Therefore, the hard mask film needs to have a certain thickness.
If the hard mask film on the select gate electrode is too thick, however, there is a problem that an opening for a contact to be connected to the select gate electrode is not satisfactorily formed when the contact to be connected to the select gate electrode is formed at the same time as a contact to be connected to the silicon substrate.